Infrared Spectroscopy Provides information about the vibrations of functional groups in a molecule Therefore, the functional groups present in a molecule can be deduced from an IR spectrum Two important parameters in all IR spectra: The frequency of the signal, The intensity of the signal, I What structural features of a molecule do and I depend on? The functional group concept of organic chemistry IR spectroscopy can identify functional groups! Some Important and Charac teristic Infrare d Absorption Frequencies and Wave lengths for Some Common Stretching Motions Atom Group Typi cal of Frequency (c m-1) Wavelength () O-H (free) O-H (H bonded) Alcoho ls (dilute) Alcoho ls (concen trated) Carboxy li c acids Acetylene (CH) Benzene ( CH), Ethy le ne (CH) Ethane (CH) Acetylene Nitr ile s Carbony l Alkene Alkane Alcoho ls , Ethers 3550-3650 c m-1 3200-3400 c m-1 2.8 3.0 3300 cm-1 3010-3100 c m-1 3.0 3.3 2950-3000 c m-1 2100-2260 c m-1 2000-2300 c m-1 1650-1750 c m-1 1620-1680 c m-1 600-1500 c m-1 1000-1300 c m-1 3.5 4.5 4.5 5.5- 6.0 6.0 6.7-17 10-7.7 C C C C H H C_____C_____H C C C C C C C N O C C O In gen eral we will only be using the d ata in an IR spectrum for stretching v ibration s which have ene rgies higher than 1620 cm-1. Although the b ands a t lower ene rgy a re known and assigned , the region b elow 1620 c m-1 is very conge sted with single bond stretche s of two heavy atoms (see C-C and C-O in table) and C-H bend s and a re beyond the scope of wha t we want to do. n-Hexane: CH3CH2CH2CH2CH2CH3 ? 1650 cm-1 3100 cm-1 1-Hexene: CH2=CHCH2CH2CH2CH3 n-Hexane: CH3CH2CH2CH2CH2CH3 3-heptanone: H3C CH2 CH2 O CH2 CH 3 CH2 n-Hexane: CH3CH2CH2CH2CH2CH3 1-Hexanol: CH3CH2CH2CH2CH2CH2OH What in the world is this peak due to? Where are the saturated C-H stretches? CH3CH2CH2CH2CH2CH3 n-hexane Where are the saturated C-H stretches? CH3CH3 H3C C C CH3 H H 2,3-dimethyl butane What’s this? Where are the C-H stretches? CH2 CHCH2CH2CH2CH3 Where are the =C-H stretches? Where is the C=C stretch? 1-hexene Why are the =C-H stretches of lower intensity than the saturated C-H stretches? Where is the the C N stretch? CH3CH2CH2CH2CH2CH2CH2C N 1-heptyl cyanide Where is the C C stretch? Where is the C-H stretch of C C H ? HC CCH2CH2CH2CH2CH3 1 - he p tyne Where is the O C stretch? O CH3CH2C CH2CH2CH2CH3 3 - he p tan o n e Why is the C-H stretch so small compared to the other spectra? Where is the O C stretch? O H3C C OCH2CH3 EthylAce tate What in the world is this peak due to? Where is the O stretch? C O CH3CH2CH2CH2CH2CH2C H 1 - h e p tan al Where is the OH stretch? Where is the C=O stretch? CH3CH2CH2CH2CH2CH2OH 1 - h e p tan o l Where is the OH stretch? Where is the O O C stretch? CH3CH2CH2CH2CH2CH2COH 1 - He p tano licAcid IR: Masses, Atoms and Springs A Model: Picture the atoms of a diatomic molecule as point masses connected by springs (bonds). As a first approximation use Hooke’s Law F = -kx F = force, restoring back to equilibrium position k = characteristic stretching constant x = displacement from the equilibrium position IR Stretching Frequencies of two bonded atoms: What Does the Frequency, , Depend On? 1 2 = frequency k mr m1 m2 mr m1 m 2 k = spring strength (bond stiffness) mr = reduced mass (~ mass of largest atom) Vibrations, potential energy and motion X Y X a F = d PE/ dr Y b X PE Y X c d re re Internuclear separation Internuclear separation r X Y Light atoms and / or strong bonds Y X Y Heavy atoms and /or weak bonds IR Stretching Frequencies: What Do they Depend On? Directly on the strength of the bonding between the two atoms ( ~ k) Inversely on the reduced mass of the two atoms (v ~ 1/m) Expect: will increase with increasing bond strength (bond order) and decreasing mass Examples of stretching frequencies and correlations with bond strengths (bond order) Bond strength* Bond order C=C C 1 1000 cm-1 600 2 1600 cm-1 840 3 2200 cm-1 350 C-C C *In kJ/mol For same reduced mass! Quantum mechanics: The frequency () depends on the energy gap between vibrational levels E = h= hc/(cm-1) Only the natural frequency will be absorbed The natural frequency (8.67 x 1013 s-1) is absorbed selectively What does the absorption intensity depend on? The absorption intensity depends on how efficiently the energy of an electromagnetic wave of frequency can be transferred to the atoms involved in the vibration The greater the change in dipole moment during a vibration, the higher the intensity of absorption of a photon Dipole Moment Must Change during for a vibration to be “IR active”! • In order to interact strongly with the EM radiation, the motion of the molecule must be such that the dipole moment changes. Which of the vibrations are “IR” active”? What is the intensity of an IR signal of: O2 or N2 or H2? Ans: In order to absorb infrared radiation, a molecular vibration must cause a change in the dipole moment of the molecule O2, N2 and H2 DO NOT ABSORB IR LIGHT! The are not “Greenhouse” gases Does O=C=O absorb IR light? Ans: vibrations of O=C=O which cause a change in the dipole moment of the molecular absorb IR light vibrations of O=C=O which do not cause a change in the dipole moment of the molecular DO NOT absorb IR light No dipole Dipole generated generated Which should have a higher stretching frequency, CO, CO+, or CO-? Why? Ans: The higher the bond order, the stronger the bond and the higher the frequency for the IR stretch. CO = ()2()4 CO+ = ()2()3 CO- = ()2()4()1 Bond order: CO = 3, CO+ = 5/2, CO- = 5/2 CO will have the higher stretching frequency CO+ and CO- will have similar, lower frequencies IR Stretching Frequencies of two bonded atoms: What Does the Frequency, , Depend On? 1 2 = frequency k mr m1 m2 mr m1 m 2 k = spring strength (bond stiffness) mr = reduced mass (mass of largest atom) The influence of reduced mass on the stretching frequency For any X-H bond the reduced mass is approximately equal to the mass of the very light hydrogen atom = 1 For a C-C bond the reduced mass is approximately equal to the mass of a carbon atom = 12 From the formula for , compute the differences in for a X-H and C-C bond of equal strengths. Infrared Spectroscopy: further review region of infrared that is most useful lies between 2.5-16 m (4000-625 cm-1) depends on transitions between vibrational energy states stretching bending Stretching Vibrations of a CH2 Group Symmetric Antisymmetric Bending Vibrations of a CH2 Group In plane In plane Bending Vibrations of a CH2 Group Out of plane Out of plane Infrared Spectrum of Hexane bending C—H stretching bending bending CH3CH2CH2CH2CH2CH3 3500 3000 2500 2000 1500 1000 Wave number, cm-1 Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 500 Infrared Spectrum of 1-Hexene C=C H—C C=C—H H2C=C H2C=CHCH2CH2CH2CH3 3500 3000 2500 2000 1500 1000 Wave number, cm-1 Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 500 Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (single bonds) sp C—H 3310-3320 sp2 C—H 3000-3100 sp3 C—H 2850-2950 sp2 C—O 1200 sp3 C—O 1025-1200 Infrared Absorption Frequencies Structural unit Frequency, cm-1 Stretching vibrations (multiple bonds) C 1620-1680 —C C— 2100-2200 —C N 2240-2280 C Infrared Absorption Frequencies Structural unit Frequency, cm-1 C O Stretching vibrations (carbonyl groups) Aldehydes and ketones 1710-1750 Carboxylic acids 1700-1725 Acid anhydrides 1800-1850 and 1740-1790 Esters 1730-1750 Amides 1680-1700 Infrared Absorption Frequencies Frequency, cm-1 Structural unit Bending vibrations of alkenes RCH CH2 910-990 R2C CH2 890 cis-RCH CHR' trans-RCH R2C CHR' CHR' 665-730 960-980 790-840 Infrared Absorption Frequencies Structural unit Frequency, cm-1 Bending vibrations of derivatives of benzene Monosubstituted 730-770 and 690-710 Ortho-disubstituted 735-770 Meta-disubstituted 750-810 and 680-730 Para-disubstituted 790-840 Infrared Spectrum of tert-butylbenzene Ar—H C6H5C(CH3)3 H—C 3500 3000 2500 Monsubstituted benzene 2000 1500 1000 Wave number, cm-1 Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 500 Infrared Absorption Frequencies: functional groups Structural unit Frequency, cm-1 Stretching vibrations (single bonds) O—H (alcohols) 3200-3600 O—H (carboxylic acids) 3000-3100 N—H 3350-3500 Infrared Spectrum of 2-Hexanol H—C O—H CH3CH2CH2CH2CHCH3 OH 3500 3000 2500 2000 1500 1000 Wave number, cm-1 Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 500 Infrared Spectrum of 2-Hexanone CH3CH2CH2CH2CCH3 O H—C C=O 3500 3000 2500 2000 1500 1000 Wave number, cm-1 Francis A. Carey, Organic Chemistry, Fourth Edition. Copyright © 2000 The McGraw-Hill Companies, Inc. All rights reserved. 500